The invention relates to a method and device for reading a bar code on a substantially motionless medium.
By substantially motionless medium is meant, in the present description, a medium which is motionless or moving relatively slowly with respect to the reading device.
Bar codes are now well known and applied in different fields : automatic mail sorting, processing of bank checks, automatic identification of products sold in stores at the time of paying at the cash tills.
When the bars of the code have a relatively large height (case of product labels), the codes may be read by moving an optical pen by hand over the label or by moving the product in front of the reading eye. The surface to be read is very limited and it is not necessary for the direction of movement to be strictly or even approximately perpendicular to the bars of the code, it is sufficient for it to intersect all the bars.
On the other hand, when the bars of the code are of a small height and when the length of the code is relatively large (bar codes of letters and checks), it is indispensable for the direction of movement of the code with respect to the reading eye to be substantially perpendicular to the bars of the code. For this reason, the objects bearing the codes to be read are often passed at high speed in front of the fixed and static reading eye.
Automatic postal sorting installations work on this principle.
A bar code called "indexation" code in France and corresponding to the postal destination code is printed with fluorescent bars on the face of the envelope or packet which bears the address. The bars have a typical height of 4 mm, a width of 0.4 mm and are separated from each other by distances equal to 1.66 mm or a multiple of this spacing. The "routing" codes occupy about 45 mm and comprise 20 bars divided into five groups each comprising a "start" bar and three bars corresponding to a figure between 0 and 9, in a "3 out of 5" code (the three bars occupying three positions out of five possible ones).
Envelopes carrying such bar codes travel at high speed (e.g. 3 m/s) in front of a reading device which must identify the code accurately and extremely rapidly. The devices used for that are generally of the type described in the French patent 2 441 889. They project a narrow light beam (for exciting the fluorescent material of the bars) having a height of 20 to 30 mm, over the zone where the indexation codes pass. The light beam reflected by the travelling envelopes is received by a photodetector associated with filters so that it is sensitive solely to the fluorescence spectrum of the bars and the amplitude peaks corresponding to detection of the bars are recorded and processed by electronic and data processing means.
The results obtained are extremely good, since the rejection rate of envelopes is less than 2.5 per 1000 despite the faults of printing the fluorescent bars forming the indexation codes.
It is now apparent that it would be desirable to be able to read such codes, not only on postal envelopes fed at high speed into the automatic sorting machines but also the piles of envelopes formed at the outlet of such machines. These piles will in fact form bundles of letters having the same destination and a routing label will have to be printed and affixed to each bundle.
Reading of an indexation code on the upper envelope of a pile raises a number of problems which cannot be solved by known devices.
In fact, these piles are either motionless or are moving slowly at the time of reading. The portions of the envelopes bearing the indexation codes occupy different positions and have different orientations with respect to the reading device.
Paradoxically, it is therefore much more difficult to read the indexation code of a motionless envelope than that of an envelope moving at high speed.
The purpose of the invention is in particular to overcome this problem.
It provides a method and device for reading a bar code on a substantially motionless medium which can be applied not only to reading the indexation codes of postal objects but also that of bar codes provided on product labels sold in stores, on checks and other documents.
It also provides a method and device of this type which are extremely precise.
It has further as object a method and device of this type which are relatively cheap and whose implementation does not require an extremely expensive investment.